U.S. Pat. No. Re. 29,448 discloses methods and apparatus for inertial spin welding of thermoplastic container parts. As disclosed in that patent, two axially mating thermoplastic container parts are respectively mounted upon axially aligned mandrels. One of the mandrels is temporarily coupled to a rotary drive means to bring that mandrel and the container part carried by the mandrel up to a predetermined rotative speed, at which time the rotary drive is disengaged, the rotary inertia developed maintaining the mandrel in rotation after the drive is disengaged. The two mandrels are then moved toward each other and the two container parts carried by the respective mandrels seat with each other. The friction developed by the relatively rotating container part heats the plastic material as it simultaneously brakes the relative rotation to melt the material to fuse the parts to each other when the relative rotation ceases and the parts are permitted to cool.
In U.S. Pat. No. Re. 29,448, the two parts being welded were both of a thermoplastic material. This fact is worthy of note in that in order to generate the frictional heat required to melt the plastic material, the mating parts of the container must fit with each other with an interference fit. Where both parts are formed from the same thermoplastic material, the achievement of an interference fit of this type is not especially difficult in that the part dimension is quite accurately established in the forming machine and any subsequent dimensional changes due to thermal expansion or contraction where the parts are stored for any substantial period of time prior to assembly normally affects both of the parts to substantially the same degree.
In recent years, there has been substantial usage of containers in which the container body or side wall is formed primarily of paperboard or cardboard, usually sealed at the opposite ends by metal tops and bottoms. Cans for motor oil and frozen orange juice are typical examples of containers of this type. Where a paperboard container body is employed, it is necessary to coat or line the interior of the paperboard body with some liquid tight material, thermoplastic materials being frequently used for this purpose.
Where the paperboard container body is lined with thermoplastic material, it has been proposed to employ a thermoplastic material for the container bottom which has led to the discovery that such bottoms could be spin welded to the container body inasmuch as the container body has a layer of thermoplastic material on its interior surface. However, difficulties have been encountered in forming and maintaining the thermoplastic coated paperboard bodies within dimensional tolerances acceptable for such a spin welding operation. In order to apply the thermoplastic liner to the paperboard, the thermoplastic is normally heated in order to bond it to the paperboard and subsequent cooling tends to shrink the material so that the container becomes undersized. Non-uniform shrinkage in storage also tends to occur, and the paperboard containers may, during preassembly handling and conveying operations, become slightly out of round.
The present invention is especially directed to methods and apparatus enabling the spin welding of thermoplastic bottoms to thermoplastic lined paperboard containers.